Immunoregulatory T cells (Tregs) play an important role in the induction of immune tolerance in autoimmune diseases. This has clearly been shown in experimental allergic encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). 3G11 is a molecule that has been identified as a sialylated carbohydrate antigen expressed on a cell membrane disialoganglioside. 3G11 is predominantly expressed on CD4+ T cells. Our preliminary data suggest that the loss of 3G11 is a characteristic of autoantigen-induced Treg cells. At present, this 3G11-CD4+ population is not fully characterized; the origin and mechanisms underlying the generation of these T cells in tolerance are not known; and the relationship between 3G11-CD4+ T cells and other Treg populations, e.g., CD4+CD25+, is not clear. The goal of this proposal is to further characterize and study the mechanisms of action and generation of 3G11-CD4+ T cells in intravenous (i.v.) tolerance. We propose the following Specific Aims: 1) To investigate the mechanism of immunoregulation by 3G11-CD4+ T cells in i.v. tolerance. We have shown that Treg cytokines like IL-10 are involved and we will test the hypothesis that cell-cell contact is also necessary for the function of 3G11- T cells. 2) To determine the mechanisms underlying the generation of 3G11-CD4+ T cells in i.v. tolerance. We will investigate the origin of these cells and test the hypothesis that they are thymus- dependent and further expand in the periphery upon tolerance. 3) To determine the relationship between 3G11-CD4+ T cells and CD4+CD25+ T cells as Tregs. We will test the hypotheses that a) 3G11-CD25+ cells are naturally occurring Tregs, while CD25+3G11+ are non-Treg, activated T cells; b) 3G11-CD25- cells are Tregs induced by i.v. tolerance; and c) FoxpS plays an essential role in the generation of 3G11- Tregs. These studies have the potential to establish a novel surface markerfor Tregs, to further enhance our understanding of the mechanisms underlying the generation of these cells, and could lead to the development of novel immunotherapies for autoimmune diseases.